A copolymer is a polymer derived from two or more monomeric species. Copolymers can be classified based on how the copolymer constituent units are arranged along the chain, including block copolymers and graft copolymers. Block copolymers comprise two or more homopolymer subunits linked by covalent bonds. Graft polymers are a branched copolymer in which the side chains are structurally distinct from the main chain.